Replacement carrier system



June 25, 1940., R. c. cuRTls REPLACEMENT CARRIER sYsTEu Filed Deo. 2l, 1957 2 Sheets-Sheet 1 INVENTOR June 25, 1940 R. c. cuRTls REPLACEMENT CARRIER SYSTEM Filed Dec. 21, 1937 INVENTOR i GCM/:bw BY ggvf n l ATTORNEY Patented June 25, 1940 REPLACEMENT CARRIER SYSTEM Richard C. Curtis, East Orange, N. J., aslignor to Wired Radio, Inc., New York, N. Y., a corporation of Delaware Application December 21, 1937, Serial No. 180,972

7 Claims.

This invention pertains in general to signalling systems and specifically relates to an arrangement for transmitting and receiving modulated high frequency carrier energy with the carrier frequency itself suppressed or eliminated.

The principal object of the invention comprises providing a system and method of suppressed carrier transmission for generating modulated carrier energy and transmitting the 10 same with the carrier frequency itself suppressed, and then, at the receiver, generating an independent carrier for replacement purposes and detection at the reception point to reproduce the desired signal with accuracy and without 1I noticeable distortion.

A further object of the invention consists in producing a system for transmitting a plurality of suppressed carrier program channels to a receiving point with means at the receiving point m' for accurately generating and replacing any one of several harmonically related carrier frequencies to reproduce a desired program.

Another object comprises providing a replacement carrier system having a receiver control frequency generating source for controlling an intermediate replacement carrier frequency source operable at selected frequencies to provide replacement carrier frequencies in the reception and reproduction of suppressed carrier program channels at selected frequencies.

These and other objects will be apparent from the following, reference being had to the accompanying drawings in which like reference numerals designate corresponding parts, and in which: A

Fig. l is a schematic representation of one embodiment of a transmission and reception sysltemof the replacement carrier type in accordance with the invention; and

Fig. 2 is a diagrammatic representation of the details. of circuits employed in the reception system of the arrangement depicted in Fig. l.

'The invention contemplates generating a base frequency and `developing a plurality of harmonies therefrom, these harmonics being utilized in conjunction with various program modulation frequencies, to produce a plurality of modulated high frequency side bands with the carrier frequencies suppressed. In accordance with the invention, these side bands of frequencies are transmitted to a reception system. The reception system comprises a control oscillator for generating a controlling frequency, nominally of a high order. 'Ihis high order frequency is then 56 utilized to interlock with a selectively operable intermediate oscillator adapted to thereby selectively produce a multiplicity of replacement carrier frequencies which are utilized in a mixing and detecting system to reproduce selected signals of programs as transmitted by the various side bands of frequencies.

Referring to the drawings in detail, and particularly to Fig. 1, the transmitter in the present embodiment of the invention comprises a crystal controlled base frequency supply source I, which may be a crystal controlled electronic oscillator, or the like, for supplying a standard base frequency, such as 10 kilocycles. The output of the base frequency source l is directed to a harmonic generator 2 which generates harmonics of the base frequency. In the present embodiment, these harmonics are delivered to modulators 3, l, 5, as frequencies of 30, 50, and 60 kilocycles, respectively.

Sources of modulation frequencies 8, 9, I0 provide various modulation frequencies to the modulators 3-5, individual to three program channels. 'I'he modulators 3-5 combine the respective carrier frequencies and modulation frequencies to produce high frequency energy in the form of modulation side bands with carriers substantially suppressed. Filters I3, I4, I5 restrict the output of the modulators 3-5 after passing through'intermediate amplifiers, so as to passone side band of desired width and substantially suppress the other side band. The spacing and arrangement of transmitted side bands may be substantially similar to those depicted in my copending applicationv Serial Number 180,971, filed concurrently herewith, Patent No. 2,188,499,v issued- January 30, 1940.

The outputs of iilters I3-I5 are directed through power amplifiers, not shown, and thence through a composite filter 23 and an integration network 24 to a transmission medium 25. The transmission medium 25 may be of various forms, as, for (example, in wired radio broadcasting, it may be a power transmission network.

The composite filter network 23, and the integration network 24, vcooperate to produce the conjoint and integrated distribution of the various frequencies over the transmission medium. 'I'he detailed construction and operation of these circuits is disclosed in a copending application of Henry R. Butler, Serial No. 161,389, filed August 28, 1937, Patent No. 2,179,612 issued November 14, 1939.'

The reception system includes a power supply 30 connected to the transmission medium 25 for supplying operating voltages over the diagrammatically indicated network Il so that the various electron-tube Voperated, equipment of the receiver may function properly.

According to the invention, a crystal controlled oscillator 3i produees'a constant wave of a high order frequency, such as kilocycles. this being the reception control frequencyvwhich, by crystal control, is maintained substantially equal to the nfteenth harmonic oi' the 10 kilocycle base frequency which also is crystal controlled at source I. A variation of a few cycles per second from the true harmonic relation will not produce noticeable distortion in the received program. Such distortion would be a change in pitch vof the received tones or notes equal to the number of cycles that the replacement carrier differs from-the original suppressed carrier and therefore is noticeable, if at all, only at very low audio frequencies. 'Ihe output of oscillator II is fed to an intermediate oscillator il. Ihe oscillator 36 has adjustable parameters and is adapted to produce frequencies over the range of carrier frequencies employed at the transmitter. I have discovered that the oscillator 34 will interlock with the control frequency of oscillator I5 and be held constant thereby when oscillator 36 is adjusted to approximately a multiple of the base frequency. 'I'he frequency of oscillator 36 can thus be maintained stable and substantially equal to any desired one of the carrier frequencies suppressed at the transmitter, the possible difference therebetween being governed by the accuracy of the crystal controls of oscillators l and 3l. The output of the oscillator 36, comprising a controlled replacement carrier thus substantially corresponding in frequency to any desired one of the carrier frequencies developed by the harmonic generatorl 2, is directed 'to a mixer 31. The mixer Il includes capacitive coupling 38h to the transmission medium 25, and derives therefrom the signalling energy comprising the modulation side bands previously discussed. These modulation side bands are then selected by the mixer 31, fed to detector 38 and amplifier 39, and then to the utilization or load circuit for reproduction of a program.

Referring to Fig. 2, it will be seen that the oscillator 35 comprises a crystal 40 associated with an oscillator circuit including electron tube 4|. The electron tube 4| is of a multi-electrode type such as the 6J? and includes necessary circuit components to form a crystal controlled oscillating circuit adapted to produce a relatively high order control frequency. In the present embodiment of the invention, this control frequency is 150 kilocycles.

The intermediate oscillator I8 includes a multielectrode tube 45 which, in the present embodiment, is of the 6L? type. 'I'he circuits connected with the tube 45 include an input connection to the control electrode from the output suppressor electrode circuit of the oscillator Il.'

The intermediate control electrode of the tube 45 is included in an adjustable resonant circuit 46 coupled with an inductance in the anode circuit 41.

Coils 49 and 50 are provided, as shown, conductively connected with a fixed capacitance Il and variable capacitor 52. In the present embodiment of the invention, I have found that the inductance 50 of a value of 16 mh. is satisfactory, while the capacitance 5I may be 280 mmf. A connection 53 from the coil II is directed to the mixer Il which includes a broadly tuned circuithavingpartsi'laandileontbeinputand output sides of electron tube 1b. 'Ine electron tube IIb is of the multi-electrode type, a ULT being here emnlvred. 'Ihe circuits lfla and 81e include variable capacitors, all of which are operated in unison for producing a broad tuning effect for selection oi' frequency bands transmitted over the lines 2l. The output of mixer I1 is directed to a substantially linear detector Il using a full wave tube Ila, type BHG being found satisfactory. The tube lla provides for the sum and difference linear detection of the frequencies delivered thereto, its output circuit being directed to the amplifier II.

The ampiiner 8l includes an ampliner tube a and frequency cut-olf circuit lsb. The tube Ila may be of the type 605.11m circuit lib is adapted to paas a definite range of audio frequencies and have a cut-off at a predetermined upper limit, for example, at 9 kilocycles. The

Y cooperative function of the mixer Il, detector Il and amplifier Il, including circuit IIb, with respect to the selection and detection of the frequencies, may be substantially the same as set forth in my above-mentioned Patent No. 2,188,499.

From the foregoing it will be seen that I have provided an arrangement for independently generating substantially synchronized equal carrier frequencies at transmission and reception points, the transmission carrier being signal modulated and transmitted with the carrier frequency suppressed, to the reception point where the transmitted andreception energies are combined for signal detection purposes.

Although a preferred embodiment of the system has been set forth, it will be understood that various modifications and changes can be made by those skilled in the art, but which will not depart from the intended scope of the invention. Therefore, no limitation is intended except as pointed out in the appended claims.

What is claimed as new and original to be secured by Letters Patent of the United States is:

1. A replacement carrier system for a series of carrier-suppressed side bands the respective carrier frequencies of which are harmonics of a constant base frequency at the transmission point, said system comprising at a receiving point, a local source of master control frequency, meansindependent of the transmitter for maintaining said control frequency constant at substantially an integral multiple of said base frequency whereby each said carrier frequency has a plurality of factors in common with said control frequency, a local oscillator selectively tunable to the frequencies of said carriers, and means including said control frequency source for maintaining the frequency of said oscillator constant and substantially equal to any one of said carrier frequencies 'when the oscillator ls tuned thereto.

12. A replacement carrier system as denned'by claim 1 in which the source of master control frequency comprises a plural-grid electron tube and a crystal having a natural frequency of vibration whichis substantially a high-order harmonic of the base frequency, said crystal being connected between two grids of said tube.

3. A replacement carrier system as defined by claim 1 in which the source of master control frequency and the selectively tunable oscillator each comprises a plural-grid electron tube, a crystal connected between two grids of the source tube, and a conductive connection from'one said grid to a control grid of the oscillator tube.

4. In the art of signalling by transmitting only side bands of suppressed carriers which are harmonically related to a constant base frequency, the method for selectively replacing carriers on such side bands which comprises, generating at a receiving point a constant control frequency substantially equal 'm cycles per second to a high order harmonic of said base frequency, generating oscillations having approximately the frequency of a selected one of said suppressed carriers,` and stabilizing the frequency of said oscillations by said constant control frequency so that the oscillation frequency is alsoconstant and bears substantially the same harmonic relation to the base frequency as the frequency of the selected carrier. l

5. In the art of signalling by single side bands having their carriers suppressed, said carriers being low order harmonics of a constant base frequency at the transmitter, the method for replacing carriers at a receiving point which comprises, generating a control frequency at said receiving point and keeping it constant at substantially a high multiple of said base frequency, generating a replacement carrier approximately equal in frequency to a desired one of the suppressed carriers, and controlling the exact frequency of the replacement carrier and keeping vit constant by said control frequency so that the replacement carrier is maintained at substantially the same frequency as the desired suppressed carrier.

6. In a receiver for suppressed-carrier side bands the carrier frequencies of which are suitable for transmission over wires and are harmonically related to a constant base frequency at the transmitter, the method for providing replacement carriers which comprises, selectively generating at a receiving point sustained oscillations approximately equal in frequency toa desired one of the suppressed carriers, generating a frequency higher than all said carrier frequencies, maintaining said high frequency con- Y stant at substantially a multiple of said constant base frequency, and stablizing said selectively generated voscillations by said' high frequency whereby the oscillation frequency is maintained.

constant at substantially the selected lower multiple of the base frequency corresponding to the desired carrier.

"1. In a receiving systemfor a group of carrier-suppressed side bands the carrier frequencies of which are harmonically related to a constant base frequency, a sourceof replacement carriers comprising, a plural-grid electronA tube, a crystal connected between two grids of said tube, means for biasing one said grid positive and the other grid negative with respect to the tube cathode,y

said crystal having a constant frequency of vibration which is substantially a high order harmonic of said base frequency, an electron tube 

